Device for optically scanning a record carrier

ABSTRACT

A watermark detector is disclosed to judge whether multimedia content can be copied or not. The watermark detector examines the multimedia content and outputs a signal indicating whether a watermark is present or not. A decision variable indicating to which extent the watermark is present is determined, for example, the amount of correlation between the input signal and a reference copy of the watermark to be detected. The watermark is detected if the decision variable exceeds a predetermined threshold (y 2 ). The detector also generates a random output signal for a predetermined range of decision values between the threshold (y 2 ) and a further threshold (y 1 ).

FIELD OF THE INVENTION

The invention relates to a method and arrangement for detecting awatermark embedded in an information signal. The invention also relatesto a method of removing a watermark from an information signal having anembedded watermark.

BACKGROUND OF THE INVENTION

Watermarks are perceptually invisible messages embedded in informationsignals such as multimedia material, e.g. audio, still pictures,animations or video. Watermarks can be used to identify the copyrightownership of information. They allow a copyright owner to trace illegalcopies of his material by inspecting whether his watermark is present insaid copies.

Watermarks are embedded in an information signal by modifying datasamples of the signal (e.g. audio samples of an audio signal, pixels ofan image, transform coefficients of a transform-coded signal, etc.) suchthat the original is not perceptibly affected. Various methods ofwatermarking are known in the art. For example, pixels of an originalimage are slightly incremented or decremented in accordance withcorresponding bits of a binary watermark pattern.

In order to detect whether an information signal has an embeddedwatermark, the signal is subjected to a statistical analysis. Thestatistical analysis yields a parameter, hereinafter referred to as“decision variable”, which indicates to which extent the watermark ispresent in the signal. For example, if an image signal is watermarked byincrementing or decrementing its pixels in accordance with a watermarkpattern, the decision variable may be the amount of correlation betweenthe signal and an applied reference copy of the watermark. If an imageis watermarked by modifying selected pixels, a prediction for saidpixels is calculated from temporally or spatially adjacent pixels. Thedecision variable may then be the number of pixels being sufficientlydifferent from their prediction.

Prior art watermark detectors generate a binary output signal indicating“watermark found” or “no watermark found”. This is achieved by comparingthe decision variable with a predetermined threshold. If the value ofthe decision variable exceeds the threshold, the watermark is consideredto be present in the signal. In consumer products such as homerecorders, the watermark detector will generally be implemented as atamperproof box, so that an attacker can neither reversely engineer thedetection algorithm nor its implementation parameters. It has beenfound, however, that an attacker can nevertheless remove a watermark byobserving the detector's binary output signal under various input signalconditions.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and arrangement fordetecting a watermark which is less vulnerable to attacks.

To this end, the method in accordance with the invention ischaracterized by the step of randomly generating said output signal fordecision values below said threshold. Preferably, the random outputsignal is generated for a range of decision values between saidthreshold and a further predetermined threshold.

The invention is based on the recognition that the prior art watermarkdetectors exhibit a sharp transition between the decisions “watermarkfound” and “no watermark found”. This property allows an attacker toiteratively modify an input signal and observe the detector's outputuntil he has found an input signal which causes the detector to operatein the vicinity of its threshold. Having thus found the transitionpoint, it is not difficult to generate an input signal which closelyresembles the watermarked signal but is not recognized as beingwatermarked. By randomizing the transition point of the detector, theattacker acquires less (or at least less reliable) information from eachsignal modification.

Further advantageous embodiments of the invention are defined in thedependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art system comprising a watermark embedder and awatermark detector.

FIG. 2 shows a watermark pattern to illustrate the operation of thesystem shown in FIG. 1.

FIG. 3 shows waveforms illustrating the operation of the prior artwatermark detector shown in FIG. 1.

FIG. 4 shows a flowchart of operations for removing a watermark from awatermarked image using the prior art watermark detector which is shownin FIG. 1.

FIG. 5 shows waveforms illustrating the operation of the watermarkdetector in accordance with the invention.

FIGS. 6-8 show embodiments of watermark detectors in accordance with theinvention.

FIG. 9 shows waveforms illustrating the operation of the watermarkdetectors shown in FIGS. 7 and 8.

FIGS. 10-12 show further embodiments of watermark detectors inaccordance with the invention.

DESCRIPTION OF EMBODIMENTS

The invention will now be described with reference to a watermarkdetector in which the decision variable indicating to which extent thewatermark is present in the signal is the amount of correlation betweenthe signal being analyzed and a reference copy of the watermark to bedetected. However, the description should not be interpreted asrestricting the invention to such an embodiment.

FIG. 1 shows a prior art system comprising a watermark embedder 1 and awatermark detector 2. The watermark embedder receives an originalinformation signal p and a watermark signal w. The information signal pis assumed to be a digitized image having 8-bit luminance pixel valuesp(n). The watermark w is assumed to be a specific binary pattern ofvalues w(n)=1 or w(n)=1. An example of such a watermark pattern is shownin FIG. 2. The watermark embedder comprises an adding stage 10 whichadds the watermark values w(n) to the spatially corresponding pixelsp(n) of the input image. It will be appreciated that this does notaffect the visual appearance of the image. The embedded watermark isthus perceptually invisible.

The information signal q is applied, after transmission or storage (notshown), to the watermark detector 2. The watermark detector comprises amultiplication stage 21 and a summing circuit 22 which jointlyconstitute a correlation circuit. The multiplication stage receives theinformation signal q and a reference copy of the watermark w, thepresence of which in the signal q is to be detected. The pixel valuesq(n) of the received image and the corresponding values w(n) of thereference watermark are individually multiplied and then summed up toobtain a decision variable y which represents the amount of correlationbetween input signal q and watermark w. In mathematical notation:$y = {\sum\limits_{n = 1}^{N}\quad {{w(n)} \times {q(n)}}}$

in which N is the total number of pixels.

The correlation value y is applied to a comparator 23 for comparisonwith a threshold value y_(thr). As is shown in FIG. 3, the comparatorproduces an output D=1 (watermark found) for y>y_(thr) and an output D=0(no watermark found) for y<y_(thr). The watermark pattern w and thethreshold value y_(thr) are carefully chosen in order to prevent thedetector from making a false decision too frequently.

A method of removing the watermark from a watermarked image using theabove described prior art watermark detector will now be described withreference to a flowchart of operations which is shown in FIG. 4. Theattack applies to any watermark detector having a sharp transitionbetween the decisions “watermark found” and “no watermark found”.

In a first step 11, a test image is created which is near the boundaryof the watermark to be removed. At this point it does not matter whetherthe resulting image resembles the original or not. The only criterion isthat minor modifications of the test image cause the detector to respondwith “watermark found” or “no watermark found” with a probability thatis sufficiently different from zero or one. The test image can becreated by tampering with a watermarked image (for which y>>y_(thr))step-by-step until the detector responds with “no watermark found”. Onemethod is to gradually reduce the contrast in the image just enough todrop below the threshold where the detector reports the presence of thewatermark. An alternative method is to replace more and more pixels inthe image by neutral grey. There must be a point where the detectormakes the transition from seeing a watermark to responding that theimage is free of a watermark. Otherwise this step would eventuallyresult in an evenly grey colored image, and no reasonable watermarkdetector can claim that such an image contains a watermark.

Having thus found a suitable test image, a portion of the image ismodified in a step 12, e.g. a particular pixel value is increased ordecreased, until the detector detects the watermark again. This providesinsight into how the watermark embedder modifies the value of thatpixel. Step 12 is repeated for every pixel in the image. It should benoted that instead of experimenting pixel by pixel, the attacker mayalso use another set of orthogonal modifications of the image, e.g.increasing or decreasing the DCT coefficients of a discrete cosinetransform coded image.

Knowing how sensitive the detector is to modification of each pixel, acombination of pixel values which has the largest influence on thedetector is estimated in a step 13. Then, in a step 14, the estimate issubtracted from the original marked image. It may be necessary tosubtract the estimate λ times to cause the detector to report that nowatermark is present. λ is found experimentally, and is preferably assmall as possible.

The above described process results in a new image which is notrecognized as being watermarked but only contains a minor distortioncompared to the watermarked image or compared to the original unmarkedimage. This attack works equally well if the watermark is embedded inthe DCT domain. The process can be repeated if the watermarkingalgorithm is suspected to contain non-linear, or image-dependentelements. Known simulation and search techniques, including simulatedannealing can be exploited in this iterative process.

The watermark detector in accordance with the invention is substantiallyless vulnerable to this attack. As is illustrated in FIG. 5, thedetector randomizes the transition point from D=0 (no watermark found)to D=1 (watermark found) if the decision variable y has a value in agiven interval y₁<y<y₂. Slightly modifying the applied signal while thedetector operates in this interval (step 12 in FIG. 4) now does not givereliable feedback to an attacker. Accordingly, the watermark can nolonger be estimated. The detector is less vulnerable to attacks as thedistance between the threshold levels y₁ and y₂ is larger.

Embodiments of a watermark detector having the desired property caneasily be designed by those skilled in the art. A straightforwardexample is shown in FIG. 6. In this example, the multiplication stage21, summing circuit 22 and comparator 23 are the same as shown in FIG.1. The detector comprises a further comparator 24 which compares theamount of correlation y with the lower threshold value y₁, and apseudo-random binary sequence (PRBS) generator 25 which generates arandom value R (0 or 1). A logic circuit comprising an AND-gate 26 andan OR-gate 27 combines both comparator outputs and the random value R toobtain the decision output signal D in accordance with the followingtruth table:

y > y₁ y > y₂ D 0 0 0 1 0 R 1 1 1

With the embodiment shown in FIG. 6, the interval end point y=y₂ wherethe detector switches from producing D=1 to producing D=R can berelatively easily found by an attacker. As the probability of producingD=0 in the interval is 50%, the first occurrence of D=0 while graduallyaffecting a watermarked image (step 11 in FIG. 4) is a reasonableindication of having found said end point. To alleviate this problem, afurther embodiment of the watermark detector is arranged to produce theoutput signal D in the interval y₁<y<y₂ with a (preferably smoothly)increasing probability as y becomes closer to the threshold y₂.

An embodiment of a watermark detector having such an increasingprobability function is shown in FIG. 7. The detector comprises anarithmetic circuit composed of a subtracter 28 and a multiplier 29 whichmodifies the decision variable y into a signal z in accordance with:$z = \frac{y - y_{1}}{y_{2} - y_{1}}$

The signal z is applied to the comparator 23 which receives a randomnumber r having a value of between 0 and 1 which is generated by arandom number generator 30. As can easily be understood, the detectorworks exactly as the detector shown in FIG. 6 for images having acorrelation y>y₂ and y<y₁. However, if the amount of correlation isbetween y₁ and y₂ (i.e. 0<z<1), the comparator output signal D dependson the actual value of r, while the probability of producing D=1increases linearly in accordance with:${\Pr \left( {D = \left. 1 \middle| y \right.} \right)} = \frac{y - y_{1}}{y_{2} - y_{1}}$

FIG. 8 shows another embodiment of the watermark detector in accordancewith the invention. In this embodiment, a random number r′ between 0 andy₂−y₁is generated by a random number generator 31 and added to thedecision value y by means of an adder 32. The signal y+r′ is thencompared with the threshold value y₂. As has been attempted toillustrate in FIG. 9, the comparator always produces an output D=0 fory<y₁ and an output D=1 for y>y₂, whereas it randomly produces 0 or 1 fory₁<y<y₂. Note that the probability of y+r′ being larger than y₂(resulting in D=1) is very small for values of y just above y₁, and verylarge for values of y just below y₂. Consequently, this embodimentinherently has the property of showing a linearly increasing probabilityof producing D=1 as y becomes larger. The linear probability curve isdenoted 91 in FIG. 9.

The inventors have found that the best shape of the probability functionin the interval y₁<y<y₂ is (or substantially resembles) a raised cosinefunction:${\Pr \left( {D = \left. 1 \middle| y \right.} \right)} = {\frac{1}{2} - {\frac{1}{2}{\cos \left( {\pi \frac{y - y_{1}}{y_{2} - y_{1}}} \right)}}}$

Such a probability curve (denoted 92 in FIG. 9) can be obtained byapplying an appropriate mathematical function F to the output of randomnumber generator 31. In FIG. 8, this function is performed by aconversion circuit 33 between the random number generator 31 and theadder 32.

Repeatedly applying the same input image to any one of the abovedescribed embodiments of the watermark detector, and thereby countingthe number of times the detector produces D=0 or D=1, teaches anattacker on which point of the probability curve the detector operates.FIGS. 10 and 11 show further improved watermark detectors which do notsuffer from this drawback. In these embodiments, the random generator isof a type having a seed input. The generator produces the same randomnumber whenever it receives the same seed. The seed input is derivedfrom the input image so that the watermark detector produces the sameoutput signal D whenever the same input image is applied. As a resultthereof, an attacker cannot obtain statistical information about theoperation point of the detector on the probability curve by repeatedlyapplying the same image.

In the embodiment shown in FIG. 10, the seed is derived from the inputimage by means of a circuit 34 which converts the received input image qinto a number having fewer bits. The function of circuit 34 is usuallyreferred to as “hash” function. The seed (for example, the modulo-N sumof all image pixel values) is then applied to the random generator 31.In the embodiment shown in Fig. 11, the correlation circuit (21,22) actsas the hash function. The decision value y itself is now applied to theseed input of the random number generator 31. It should be noted thatthe feature of applying a seed to the random generator can also beadopted for the embodiments shown in FIGS. 6 and 7.

The difference between a watermark detector with and without the seedfeature can best be explained by way of an example. Applying the sameinput image 100 times to a watermark detector not having the seedfeature will cause said detector to produce, for example, 90 times anoutput D=1 (watermark found) and 10 times an output D=0 (no watermarkfound). Applying the same input image 100 times to a watermark detectorwith the seed feature will cause the detector to produce 100 times thesame output, the probability of D=1 being 90% and the probability of D=0being 10%. In the latter case, an attacker cannot gather statisticalinformation by repeating the watermark test for the same image over andover again.

A watermark detector is particularly invulnerable to attacks if theabove described features (smoothly increasing probability function, seedsupply through the hash function and seed supply by the decisionvariable itself) are combined. Such an embodiment is shown in FIG. 12.

Randomization of the watermark detection point can also be obtained byrandomly selecting the pixels considered for calculating the decisionvariable y (or, conversely, the pixels that are discarded). To this end,randomly selected image pixels q(n) and corresponding watermark valuesw(n) are applied to the correlation circuit 21,22 which is shown in FIG.1. For example, if 60% of the pixels of a watermarked image areconsidered and the image is not modified by an attacker, the detectorwill still generate D=1 in spite of the decision value being less thanwhen all pixels are considered. However, if pixels of the image aremodified, the decision value can decrease which may result in D=0 beinggenerated, dependent on how many pixels have been modified.

The invention can be summarized as follows. Recently developed methodsfor copy protection rely on a watermark detector to judge whethermultimedia content can be copied or not. In such copy protectionschemes, a watermark detector examines the multimedia content andoutputs a signal (D) indicating whether a watermark is present or not.Known watermark detectors determine a decision variable (y) indicatingto which extent the watermark is present, for example, the amount ofcorrelation between the input signal and a reference copy of thewatermark to be detected. The watermark is detected if the decisionvariable exceeds a predetermined threshold (y₂). Such a detector isvulnerable to an attack which is described in this patent application.

Disclosed is a watermark detector which increases the work load for anattacker by several orders of magnitude. To this end, the detectorgenerates a random output signal for predetermined range of decisionvalues (y) between the threshold (y₂) and a further threshold (y₁).

What is claimed is:
 1. A method of detecting a watermark embedded in aninformation signal, comprising the steps of: determining a decisionvariable indicating to which extent the watermark is present in thesignal; and generating an output signal indicating detection of thewatermark if the decision variable exceeds a predetermined threshold(y₂); and randomly generating the output signal for decision valuesbelow the threshold (y₂).
 2. The method as claimed in claim 1, furthercomprising the step of randomly generating the output signal for a rangeof decision values between the threshold (y₂) and a furtherpredetermined threshold (y₁).
 3. The method as claimed in claim 1,wherein the random output signal is generated with an increasingprobability as the decision variable becomes closer to the predeterminedthreshold (y₂).
 4. The method as claimed in claim 3, wherein theprobability is a linear function of the decision variable within therange between the threshold (y₂) and the further threshold (y₁).
 5. Themethod as claimed in claim 3, wherein the probability is a raised cosinefunction of the decision variable within the range between the threshold(y₂) and the further threshold (y₁).
 6. The method as claimed in claim1, wherein the step of randomly generating the output signal includesgenerating the same output signal whenever the same information signalis received.
 7. An arrangement for detecting a watermark embedded in aninformation signal, comprising: means for determining a decisionvariable indicating to which extent the watermark is present in thesignal; and means for generating an output signal indicating detectionof the watermark if the decision variable exceeds a predeterminedthreshold (y₂); means for randomly generating the output signal fordecision values below the threshold (y₂).
 8. The arrangement as claimedin claim 7, in which the arrangement further comprises means forrandomly generating the output signal for a range of decision valuesbetween the threshold (y₂) and a further predetermined threshold (y₁).9. The arrangement as claimed in claim 7, further comprising means forgenerating the random output signal with an increasing probability asthe decision variable becomes closer to the predetermined threshold(y₂).
 10. The arrangement as claimed claim 9, wherein the means forrandomly generating the output signal includes a random number generatorwith a seed input, and means for deriving the seed input from theinformation signal in accordance with a predetermined function.
 11. Thearrangement as claimed in claim 9, wherein the means for determining thedecision variable constitute the means for deriving the seed input. 12.A multimedia playing and/or recording apparatus comprising: means forreading an information signal from and/or writing an information signalto multimedia material storage; means for determining a decisionvariable indicating to which extent the watermark is present in thesignal; and means for generating an output signal indicating detectionof the watermark if the decision variable exceeds a predeterminedthreshold; means for randomly generating the output signal for decisionvalues below the threshold (y₂).
 13. The method of claim 1, in which:the output signal is randomly generated for a range of decision valuesbetween the threshold (y₂) and a further predetermined threshold (y₁);the random output signal is generated with an increasing probability asthe decision variable becomes closer to the predetermined threshold(y₂); the probability is a linear function of the decision variablewithin the range between the threshold (y₂) and the further threshold(y₁); the probability is a raised cosine function of the decisionvariable within the range between the threshold (y₂) and the furtherthreshold (y₁); and the step of randomly generating the output signalincludes generating the same output signal whenever the same informationsignal is received.
 14. The arrangement of claim 7, in which: thearrangement further comprises means for randomly generating the outputsignal for a range of decision values between the threshold (y₂) and afurther predetermined threshold (y₁); means for generating the randomoutput signal with an increasing probability as the decision variablebecomes closer to the predetermined threshold (y₂); the means forrandomly generating the output signal includes a random number generatorwith a seed input, and means for deriving the seed input from theinformation signal in accordance with a predetermined function; and themeans for determining the decision variable constitute the means forderiving the seed input.